Kinematic thrust bearing with balls and rollers

ABSTRACT

A novel thrust bearing is disclosed in which rotation is accompanied by a minimum of axial motion by reason of the averaging of defects over the various elements making up the bearing. A first race is partially filled with about two-thirds of a full complement of balls, the number of balls being odd. These balls are spaced apart by an equal odd number of grooved rollers which are held up out of contact with the race by the balls. A second row of balls each engaging two of the rollers and an upper race complete the novel structure. The construction has no resistance to radial displacement or tilting and must be used in combination with radial bearings. The construction with an even number of rollers is unstable. The construction is shown for the introduction of controlled and very small amounts of cyclic irregularities by tilting of one race.

United States Patent 1 Howland et al.

1 1 KlNEMATlC THRUST BEARING WITH BALLS AND ROLLERS [76] lnventors:Bradford Howland, Rm. 20-C-007,

RLE-M.I.T., Cambridge, Mass. 02139; Howard C. Howland, 205 WinstonDrive, Ithaca, NY. 14850 [22] Filed: July 12, 1971 [21] Appl. No.:161,854

[52] US. Cl. 308/206 [51] Int. Cl. Fl6c 33/00 [58] Field of Search308/203, 206

[56] References Cited UNITED STATES PATENTS 490,859 1/1893 Pardon et al.308/206 1,119,034 12/1914 Parker 308/206 Primary ExaminerCharles J.Myhre Assistant ExaminerFrank Susko Attorney-Jack Larsen in] 3,751,125 11 Aug. 7, 1973 5n ABSTRACT A novel thrust bearing is disclosed in whichrotation is accompanied by a minimum of axial motion by reason of theaveraging of defects over the various elements making up the bearing. Afirst race is partially filled with about two-thirds of a fullcomplement of balls, the

number of balls being odd. These balls are spaced apart by an equal oddnumber of grooved rollers which are held up out of contact with the raceby the balls. A second row of balls each engaging two of the rollers andan upper race complete the novel structure. The construction has noresistance to radial displacement or tilting and must be used incombination with radial bearings. The construction with an even numberof rollers is unstable. The construction is shown for the introductionof controlled and very small amounts of cyclic irregularities by tiltingof one race.

10 Claims, 2 Drawing Figures um I um mwuu ""iimn 1 PAIENIEWE 1w3.151.125

FIG. I

Howard GHou/land INVENTORS.

'BY 1m ATTORNEY This invention relates to bearings and particularly to Ia novel thrust bearing in which rollers and balls work together toprovide a bearing wherein the forces between elements areself-equalizing; that is, the device is kinematic.

The prior art is replete with examples of bearings in which balls orrollers are employed as spacers between load-bearing balls. Germanpatent 205,106 to Fischer, British patents 27,268 of 1897 to Philippeand 18,383 of 1893 to Broadbent et al; and U.S. Pat. No. 366,] l7 toLake are examples of this kind of bearing.

U.S. Pat. No. 3,423,142 to Wietrzykowski, No. 943,570 to Schluss, and665,653 to Faller and German patent 174,448 (1904) to Hofert, areexamples of bearing in which rolling elements either balls or rollers,space apart other rolling elements and are also loadbearing. Thesebearings are to be distinguished from the 'present invention primarilyin that they are radial bearings in structure, which may have greater orlesser capacity for bearing axial loads, and by the fact that many ofthese patented designs are impractical in that the configuration ofrolling elements shown in the drawings is unstable in that it is subjectto spontaneous rearrangement and collapse. Surprisingly the generalstructure of the present invention is likewise unstable if itsconstruction is attempted with an even number of elements, but is stableif assembled of an odd number of elements. I

In one test of the invention an upper plate having a circular race about2.2 inches in diameter on its lower face was fastened to ,a /4 inchshaft at the center of the race and extending downward. To this shaft alower plate was journalled with appropriate radial ball bearings. Theupper face of the lower plate also had a race 2.2 inches in diameterfacing the upper race. A conventional ball thrust bearing would beformed by inserting a row of balls between these races; and the novelbearing of this invention performs substantially the function of such aconvention bearing. If in a conventional ball thrust bearing, the ballswere perfectly hard, and perfectly round, and if the races wereperfectly true and perfectly hard, and if foreign bodies would foreverbe excluded from between the elements, and if the selected means forkeeping the balls apart were likewise perfect, there would be no needfor the present invention. It is, therefore, an object of the presentinvention to reduce by averaging out, the effect of localizedimperfections in balls, races, and retaining and separating means, andthe effect of small foreign bodies in such a bearing.

The construction by which this object is attained is characterized bythe inclusion of additional layers of rolling elements. Continuing withthe above example, the races would accommodate a full complement of 24balls of V4 inch diameter. According to the invention, only 17 balls areplaced on the lower race. Then to separate these balls, 17 rollers eachwith a toroidal groove on its outer surface and about five-sixteenthsinch in diameter are positioned on the balls, the groove of each rollerengaging two successive balls on the lower race. In this example therollers were defunct inner races from conventional radial ball bearingswhich were canabalized for the purpose. Holding these in placetemporarily (with adhesive tape, for example), a second set of 17 ballsis placed each to engage the grooves of successive rollers, then theupper race is applied. It is noted that while a load may be applied tothe upper race along the axis of the bearing, it does not resisttipping, and is relatively soft" and compliant as to motion of the upperrace parallel to the lower race. The thrust bearing so constructed mustbe supplemented by other structure to restrict tippingand radial motion.

If the construction had been attempted starting with either 16 or 18balls, the assembly could be made, and a load applied; but with thefirst start of motion one roller would tend to ride up between a pair ofballs, the next roller would tend to drop down, and so on around thering with resulting complete collapse. With an odd number of rings, thisform of progressive failure one way around is cancelled by an oppositeprogression the other way, so that constructions with an odd number ofrollers are stable, while those with an even number of rollers areunstable.

With this construction, if one ball is out-of-round, the effect of thisout-of-roundness is not communicated directly to the opposing race, butonly through the interpendicular to the plane of the first race, andmakes an angle B with the perpendicular to the plane of the first race,then asone race rotates relative to the other around the axis, thedistance between the planes of the races measured along the axis variesaccording to the equation:

d= k k,AB cos 0 k sin(20 P,)

. k, sin(n0 n) I 1 where k k k k,,'are constants, 6 is the output anglemeasured from the point of maximum error. 0rdinarily only k and k aresignificant, and since the error contribution from the first order termis proportional to the product of two ordinarily small angles, this termalso is small. It is also controllable. Thus if A is fixed and verysmall, B may be adjusted in amount and phase to introduce into a systema very small sinusoidal compensation. It is, therefore, also an objectof the invention to provide means for providing small compensatorymotions in mechanical systems.

It is a further feature of the invention to provide a ball bearing inwhich all contacts may be rolling contacts and in which the ballassembly is kept tight by the thrust load without backlash; and it is afurther object of the invention to provide such anti-backlash feature;but it is not suggested that this feature is better provided by this newconstruction than by, the abovementioned prior-art constructionsspecifically and primarily directed to this feature.

Other objects and features of the invention will in part be obvious andin part be apprehended from the following description taken inconjunction with the annexed drawings of which:

surface of the base, and a pair of small spur idler gears 16 and 17 withtheir axes generally perpendicular to the surface of the base. Engagingthe worm 14, and the idler gears 16 and 17 at substantially equalintervals around their circumferences are two thin, worm gear wheels 20and 22, which in the preferred embodiment are three inches in diameterand have respectively 359 and 362 teeth. Thus the teeth of the two wormgears are in substantial alignment at only three points on theircircumferences where they engage the worm 14 and the two idlers l6 & 17.The stub shaft 28 on which one of the idlers 16 rotates is groundeccentrically so that the axis of the idler 16 does not coincide withthe hole in the base 10 in which it is inserted. This allows a very fineadjustment, which in coaction with similar eccentricity in the mountingbolt 30 for the worm block 32 permits the worm wheel 20 to be adjustedin perfect concentricity with the shaft 12. The idler stub shaft 34which carries the idler 17 has a handle 36 pinned to its end and isbushed into the base 10 with an eccentricity of about one-sixteenth ofan inch so that when the handle 36 is turned one-quarter turn, the wormwheels 20 and 22'are released.

The lower ball race 50 of the bearing is ground into the upper surfaceof the wheel 20. The bottom of the wheel 20 is ground flat with a smallwedge angle such as 95 of arc between the bottom and the effective planeof the race 50. Similarly both top and bottom of the wheel 22 are groundflat with a similar small dihedral angle between them. Thus with wheel20 on top of wheel 22, rotation of one relative to the other will effecta tilt of the race relative to the base in magnitude between and 1.Rotating both wheels together changes the azimuth of that tilt.

Since the number 359 of teeth on wheel 20 has no common factor with thenumber 362 of teeth on wheel 22, it required 362 X 359 129,958 turns toexhaust the possibilities of magnitude and direction of tilt to beintroduced into the lower race. In practice the approximately 21,660turns to rotate one wheel 180 with respect to the other will provideprecision commensurate with practical apparatus.

In the embodiment of FIG. 1, the lower race is a vee race with two linesof contact with the balls. The outer conical surface of the vee is lesssteeply inclined to the horizontal than the inner conical surface andcarries most of the load. The lines of contact on the balls form twocircles with circumferences which are proportional to the circumferencesof the corresponding lines of contact on the races, so rolling contactis maintained, and precession of balls is a second-order effect. Thereare nine balls 60 in the lower race 50 and nine rings 65 which serve asrollers. These rings are reground inner races from a radial ball bearingand their outer surfaces are a circular toroidal shape closely matchingthe curvature of the balls. The least principal radius of curvature ofthe race surface is only about greater than the radius of the balls, anda wear track is noted somewhat outside of the centerline of the rollers.The upper set of nine balls 70 in this case are of the same diameter asthe lower set, and the upper race 80 is of the same size and shape asthe lower race and is ground in the bottom surface of the end piece 82.The lower hub of the piece 82 contains a radial bearing (not shown)which engages the shaft 12 to center the hub of the piece 82 relative tothe shaft. The shaft extends through the hub, and is retained by a clip84 which holds the bearing together when the thrust load is removed, Theupper hub is threaded on its outer diameter to engage internal threadson shaft enlarged portion, 86 at the end of a precision lead screw shaft88. A second radial bearing (not shown) at the other end of the leadscrew redundantly maintains the radial stability of the thrust bearingaxis.

The end piece 82 is first assembled to the lead screw and the screwfitting stabilized with a set screw before the race is ground and thisrace and the seat for the radial bearing are ground concentric andperpendicular to the axis of the precision lead screw. The end piece isthen removed from the lead screw and the thrust bearing is assembled.The piece may again be assembled to the lead screw in a reproducableway, the threads having an interference fit that is tightened only tothe point marked by the set screw or set screws.

FIG. 2 is representative of a bearing in which balls contact each raceat two points, that is, each ball has six points of contact, two witheach of two rollers and two with the top or bottom race as the case maybe. To maintain true rolling contact at all points of contact thebearing may be designed so that all of the rolling elements roll aroundaxes that form appropriate predetermined angles with the output axis ofthe bearing. A solution is that all of these axes intersect the outputaxis of the bearing at the same point C. Since this is a symmetricalconfiguration with upper and lower races of the same diameter, thatpoint is midway between the planes of the upper and lower races, so thatthe rollers roll about horizontal axes, pointed inward, as for exampleCR. The design is reached by a succession of trials, which may be workedout graphically on drawings of a vertical section through the center ofa ball, which contains the contact points of the balls and the upper andlower races, and a slant section containing the axes about which one ofthe rollers and two of the adjacent balls roll, which contains thepoints of contact between balls and rollers. It is necessary that if thepoint E represents the point of contact between the ball and the upperrace outer conical surface 102, then the point F which is the point ofcontact between the ball 100 and the inner conical surface 104 shall lieon the line CE.

In like manner it is necessary that if the point H is the point ofcontact between the ball 100 and conical surface 112 which is the outerrace surface of the roller 114, then the point I which is the point ofcontact between the ball 100 and the conical surface 116, must lie onthe line CH. In like manner the point of contact K and L must lie on astraight line through C and points of contact M and N must also define aline through C. In an alternate similar configuration, all race groovesare of substantially the same circular cross section, having a leastprincipal radius of curvature only a few percent greater than the radiusof the balls.

Although it is preferred that the effective diameters of balls androllers be about equal, and in the given examples, the balls were all ofone size, this is not necessary, nor is it necessary that the upper andlower races have the same diameter, or that the upper row of ballsshould be of the same diameter as the lower row of balls. It is possiblefor the upper row of balls to be less than half the diameter of thelower row, in which case the upper race has a substantially smallerdiameter than the lower race and the rollers necessarily lean inward andtheir axes of rotation are inclined relative to the races. In such aconstruction the groove in the rollers would be contoured to engage thesmaller balls in contact rings which lie between the points of contactfor the larger balls which define the rings of contract for those balls.

The foregoing descriptionof an embodiment of the invention being by wayof illustration only, the scope of the invention is defined by thefollowing claims.

We claim:

1. An axial antifriction bearing comprising three series of rollingparts, between a first circular ball race open in a first axialdirection and lying in a plane perpendicular to that direction, and asecond circular ball race substantially coaxial to, and parallel to,said first race and facing it open in the opposite axial direction,wherein the first series of parts is an odd number, greater than one, ofballs engaging said first race, said second series of parts is an equalodd number of balls engaging said second race, and said third series ofparts is an equal odd number of rollers each of said rollers beingsupported by four of said balls, two balls fromsaid first series and twoballs from said second series, by the contact of said four balls with acircular groove around the outer surface of said roller. I

2. An axial antifriction hearing as defined by claim one wherein saidraces and said groove are circularly toroidal in form.

3. An axial antifriction hearing as defined by claim one wherein one ofsaid races is. circularly toroidal in form. v

4. An axial antifriction bearing as defined by claim one wherein saidgroove comprises a circularly toroidal surface for contacting at leasttwo of said four balls.

contact with said toroidal surface.

6. A bearing as defined by claim one in further combination with meansfor connecting said first race to a base, means for connecting saidsecond race to other apparatus, and radial bearing means to constrainsaid first and second races to relative rotation about their commonaxis. I

7. A bearing as defined by claim onein further combination with meansfor adjusting the tilt of one of said races relative to said first axialdirection in amount and in direction.

5. An axial antifriction bearing as defined by claim 4 wherein saidgroove comprises a pair of conical sur-- faces for contacting the pairof said four balls not'in 8. A bearing as defined by claim one whereineach of said balls is supported at six points of contact, said sixpoints of contact defining three lines for each of said balls, a firstline defined by the pair of points of contact with one of said races, asecond line defined by the pair of points of contact with one of saidrollers, and the third line defined by the pair of points of contactwith another of said rollers, all of said lines converging on a point onthe axis of said races.

9. A bearing as defined by claim one wherein the points of contactbetween said balls and said races and rollers describe on the surfacesof said balls, rollers, and races, a plurality of circles, each of saidcircles defining a line normal to its plane and through its center,wherein all of said lines are designed to converge at a point, andsubstantially so converge.

10.'A bearing as defined by claim one wherein the points of contactbetween said balls and said races and converge to a center,substantially a point. i k i 1i

1. An axial antifriction bearing comprising three series of rollingparts, between a first circular ball race open in a first axialdirection and lying in a plane perpendicular to that direction, and asecond circular ball race substantially coaxial to, and parallel to,said first race and facing it open in the opposite axial direction,wherein the first series of parts is an odd number, greater than one, ofballs engaging said first race, said second series of parts is an equalodd number of balls engaging said second race, and said third series ofparts is an equal odd number of rollers each of said rollers beingsupported by four of said balls, two balls from said first series andtwo balls from said second series, by the contact of said four ballswith a circular groove around the outer surface of said roller.
 2. Anaxial antifriction bearing as defined by claim one wherein said racesand said groove are circularly toroidal in form.
 3. An axialantifriction bearing as defined by claim one wherein one of said racesis circularly toroidal in form.
 4. An axial antifriction bearing asdefined by claim one wherein said groove comprises a circularly toroidalsurface for contacting at least two of said four balls.
 5. An axialantifriction bearing as defined by claim 4 wherein said groove comprisesa pair of conical surfaces for contacting the pair of said four ballsnot in contact with said toroidal surface.
 6. A bearing as defined byclaim one in further combination with means for connecting said firstrace to a base, means for connecting said second race to otherapparatus, and radial bearing means to constrain said first and secondraces to relative rotation about their common axis.
 7. A bearing asdefined by claim one in further combination with means for adjusting thetilt of one of said races relative to said first axial direction inamount and in direction.
 8. A bearing as defined by claim one whereineach of said balls is supported at six points of contact, said sixpoints of contact defining three lines for each of said balls, a firstline defined by the pair of points of contact with one of said races, asecond line defined by the pair of points of contact with one of saidrollers, and the third line defined by the pair of points of contactwith another of said rollers, all of said lines converging on a point onthe axis of said races.
 9. A bearing as defined by claim one wherein thepoints of contact between said balls and said races and rollers describeon the surfaces of said balls, rollers, and races, a plurality ofcircles, each of said circles defining a line normal to its plane andthrough its center, wherein all of said lines are designed to convergeat a point, and substantially so converge.
 10. A bearing as defined byclaim one wherein the points of contact between said balls and saidraces and rollers describe on the surfaces of said balls, rollers, andraces, a plurality of tracks which are substantially circles, each ofsaid circles defining a line normal to its plane and through its centerwherein all of said lines converge to a center, substantially a point.